Betting odds explained
Key takeaway
Decimal odds show total return per GBP1 staked including the stake; implied probability = 1 divided by decimal odds - the gap between all runners' implied probabilities and 100% is the bookmaker's built-in margin.
Betting odds tell you two things at once: what a bookmaker thinks the chance of something happening is, and how much you'd be paid if you're right. In the UK you'll usually see odds in one of two formats — fractional (like 5/2) or decimal (like 3.50) — and most UK bookmakers let you switch between the two.
Key facts
- Fractional odds (e.g. 5/2) show profit relative to stake: a £10 bet at 5/2 wins £25 profit (5/2 × £10), plus your £10 stake back — £35 total returned.
- Decimal odds (e.g. 3.50) show total return per £1 staked, stake included: a £10 bet at 3.50 returns £35 total (£25 profit + £10 stake back) — the same bet as above, just expressed differently.
- To convert fractional to decimal: divide the fraction and add 1. 5/2 = 2.5 + 1 = 3.50.
- Odds-on prices (e.g. 1/2, or 1.50 decimal) mean the outcome is considered more likely than not — you stake more than you can win in profit.
- Evens (1/1, or 2.00 decimal) means your profit equals your stake if you win.
Implied probability
Odds imply a probability. For decimal odds, implied probability = 1 ÷ decimal odds, shown as a percentage. Decimal odds of 3.50 imply a 1 ÷ 3.50 ≈ 28.6% chance, as the bookmaker prices it. Adding up the implied probabilities of every runner in a market will normally total more than 100% — the difference is the bookmaker's built-in margin (sometimes called the "overround" or "vig"), which is how bookmakers make money over time regardless of the outcome.
Worked example
| Fractional | Decimal | £10 stake returns | Implied probability |
|---|---|---|---|
| 1/4 | 1.25 | £12.50 | 80.0% |
| 4/6 | 1.67 | £16.70 | ~59.9% |
| Evens (1/1) | 2.00 | £20.00 | 50.0% |
| 5/2 | 3.50 | £35.00 | ~28.6% |
| 10/1 | 11.00 | £110.00 | ~9.1% |
Common misconceptions
- "Bigger odds always mean bigger risk." They mean the bookmaker considers the outcome less likely — that's usually correlated with risk, but not the same thing as risk itself.
- "Odds are a guarantee." Odds reflect a bookmaker's pricing of probability plus their margin, not a certainty. A 1/10 favourite still loses sometimes.
- "All UK bookmakers price markets identically." They don't — the same event can carry different odds and different margins at different bookmakers, which is part of why comparison is useful.
Related concepts
Odds format is separate from bet types — see our guides to each-way betting and ante-post betting for how odds apply to specific horse-racing bet structures.
Frequently asked questions
How do I convert fractional odds to decimal?
Divide the fraction and add 1. For example, 5/2 = 2.5 + 1 = 3.50 decimal.
What does implied probability mean?
It's the chance of an outcome that a given set of odds represents. For decimal odds, implied probability = 1 divided by the decimal odds, shown as a percentage.
Why do all the implied probabilities in a market add up to more than 100%?
That extra percentage is the bookmaker's built-in margin (sometimes called the overround), which is part of how bookmakers price markets to make money over time.
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